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Use Data Contouring to Find Patterns in Data

Contours are lines of constant value drawn through fields of two-dimensional data. On one side of the line will be greater values than the contour's, and on the other side, lesser values. The data field doesn't need to be horizontal, it may be oriented any way possible, including having time as one of its axes.

Fields of data yield patterns that tell stories to scientists, business people, doctors, artists, athletes, hikers, explorers, etc. Similar to drawing, contouring slows us down to study the spatial relationships in fields of data, or, in other words, the patterns in data. There are many automated tools that contour data for us, but it is helpful to understand how to contour data so we can better interpret the stories we find from the observed patterns, even when an automated process did the contouring.

Drawing a Contour Map

We are going to draw temperature contours (called isotherms = lines of constant temperature) for a vertical cross section between the Mediterranean Sea and the Atlantic Ocean which is located at the Straits of Gibraltar. Download data below.

Suggested Time for the Following 6 Steps: 30-60 min depending on comfort with drawing and working with data.

Steps 1 & 2

1) Orient yourself with the data field. At the top of box are surface temperatures. At the bottom of the box are temperatures at 1.2 km below the surface.

Data on the left are near the edge of the European side of the Mediterranean Sea, and data on the right are near Africa. Data are missing with depth, most likely due to an irregular sea floor bottom.

2) Find the maximum and minimum temperatures. In this case, the warmest temperature is 15.2°C and the coldest 10.0°C. This is a range of 5.2°C. This makes it straightforward to pick a contour interval of 1°C (the temperature between each contour).

Step 3

3) It is helpful to draw a contour for a value approximately midway between the maximum and minimum values since this usually divides the map into two somewhat equal regions. Now you are able to see patterns more readily. So draw the contour for 13°C.

Tip: if possible, start at an edge of the map. In this case, 13 is nearly halfway between 14 and 11.9 on the left edge of the map.

Tip: go through and put dots where 13 falls between sets of adjacent points Connect the dots with a smooth line. Draw lightly with pencil and have an eraser handy. Only draw where there are data - you cannot draw into a region with missing data. When done, label your contour "13".

Steps 4 & 5

4) Draw and label the rest of the contours, starting with those adjacent in value to your first contour.

Tip: If you have a good deal of contouring to do, listen to some relaxing music.

Rules for Contouring

Contours cannot cross.

Contours cannot branch or fork.

Label all contours.

No contours in areas with missing data.

5) Look for patterns and see if there are stories in the data.

Although the warmest water tends to be near the surface, there is very cold water with depth along Europe, and it gets cold very quickly with depth. Warm water with depth is found near Africa. This is very puzzling since warm water is less dense than cold water, so the coldest water should be found at the bottom of the entire field of data.

Challenge

Contour the salinity data for this same vertical cross section (use the data link above). Salinity contours are called isohalines. Compare your answers to the solution (download below), then identify which water is entering from the Atlantic Ocean and which is exiting the Mediterranean Sea.